1. Field of the Invention
The present invention relates processes for fabricating mirrors suitable for use in precision instruments such as ring laser gyroscopes. More particularly, this invention pertains to a process for controlling the thickness of mirror layers deposited upon a mirror substrate within a deposition chamber.
2. Description of the Prior Art
The ring laser gyroscope is a rotation sensor that senses rotation about an axis that is perpendicular to the plane of a cavity formed within a frame, preferably of glass ceramic or other low thermal coefficient material. Highly polished mirrors are positioned at the corners of the cavities to direct the counterpropagating beams about the cavity. Beams of laser light circulate in opposite directions within the cavity. In accordance with the well-known Sagnac effect, the frequencies of the two beams are altered in opposite senses (i.e. one is increased while the other is decreased) by rotation about the axis and the beat frequency between the two beams then provides a measure of rotation.
Lasing is affected within the cavity by the interaction of photons with an excited medium which acts as an amplifier. In a d.c. configuration, the medium is excited by the interaction of a fill gas, typically HeNe, with flows of electrical current between electrodes arranged about or within the gyro frame. Alternatively, in an r.f. actuated device, the medium is excited by means of an electromagnetic field that oscillated at radio frequencies. Only two counterrotating lasing modes need to be supported within the lasing cavity to obtain a measure of rotation. In a planar cavity, the counterrotating beams are linearly polarized whereas a nonplanar cavity can support both right and left circularly-polarized modes.
As mentioned above, mirrors are provided at the corners of the cavity for directing the beams of light. In the case of a multioscillator or other multiple-cavity device the number of mirrors or optical elements will correspondingly increase. The precision and, for that matter, operability of a ring laser gyroscope is critically dependent upon the quality of the mirrors. Surface defects and unevenness can produce a multitude of device infirmities.
The fabrication of high reflectance mirrors for use in precision instruments such as ring laser gyroscopes involves the careful deposition of multiple layers of various material compositions. Multiple layers are required to provide the high reflectivity that is necessary to generate the feedback required for ring laser gyro operation. Without the necessary high reflectivity, the gyro may be unable to assume operation as it is a low gain oscillator. That is, successful gyro operation requires that gyro gain exceed losses for the desired oscillation to occur.
The need to deposit multiple layers of material multiplies the possible sources of mirror surface nonuniformity. The presence of nonuniform mirror layers may result in the creation of a visually observable color band across the mirror aperture showing a spatial distribution of frequency response that does not permit or severely impacts laser gyro operation.
The coating layers of a precision mirror are created through a variety of deposition processes. Such processes are conventionally performed within a coating chamber that essentially comprises a sealable vacuum box. Among the processes for depositing materials in such an environment are thermal evaporation, electron beam, ion beam and magnetron sputtering. FIG. 1 is a schematic view of the application of a mirror layer by one of such deposition processes within a coating chamber. Within the chamber, mirror substrates 10 are mounted upon a generally-planar rotatable tool 12 for receiving deposited material 14. The physics of each of the above-named processes is characterized by the generation of an inherently-nonuniform spatial distribution of coating material known as a "plume".
The inherent, generally-predictable shape of the plume generated of course complicates the task of depositing mirror layers of uniform thickness. Even with rotation of the tool 12, it is clear that thicker layers will be deposited upon the mirror substrates 10 located closest to the center of the tool since the plume shape reaches a maximum in this region. This is addressed in the prior art by the use of a shadow mask 16 as shown in the more complete schematic view of a chamber deposition process illustrated by FIG. 2. The mask 16 is held rigidly within the coating chamber and suspended above the tool 12 which may undergo either simple or complex rotation. The shape of the shadow mask 16 is successively trimmed until the desired uniformity of layer deposition in the presence of the plume-like emission of material is observed. Different types of coating tooling may require different masks as the tooling itself affects the uniformity of the coatings deposited. Small process variations or system maintenance may result in a condition where the size of the shadow mask is too small. This generally requires the fabrication of a new mask and the initiation of a new trimming sequence. Such a process is inherently trial and error, time consuming and neither particularly accurate nor predictable.